This invention relates to a method and to a combination of materials for facilitating the release of a stuck pipe, particularly a differentially stuck pipe.
Stuck pipe may be defined as drill pipe, drill collars, drill bits, stabilizers, reamers, casing, tubing, measurement while drilling tools, logging tools, etc., having inadvertently become immovable in a wellbore. The term "stuck pipe" is used in the industry as a convenient compendious term to cover the sticking of all such equipment and is generally understood as not being restricted literally to pipes. Sticking may occur when drilling is in progress, when pipe and casing is being run in the hole or when a drill pipe is being hoisted.
There are numerous causes of stuck pipe; some occur regularly, some may be peculiar to a particular area and some may be unique. Industry convention categorizes the causes as either differential or mechanical sticking.
Differential sticking is believed to occur by the following mechanism. During most drilling operations, the hydrostatic pressure exerted by a drilling mud column is greater than the formation fluid pressure. In permeable formations, mud filtrate flows from the hole into the rock and builds up a filter cake. A pressure differential exists across the filter cake which is equal to the difference between the pressure of the mud column and the pressure of the formation.
When a pipe is central in the bore, the hydrostatic pressure due to the mud overbalance acts in all directions around it. If, however, the pipe touches the filter cake, the mud overbalance acts to push the pipe further into the cake, thus increasing the contact area between the pipe and the cake. Filtrate is still expelled from the filter cake between the pipe and the formation, thus shrinking the cake and allowing the pipe to penetrate further into it and so increasing the contact area still more. If the pressure difference is high enough and acts over a sufficiently large area, the pipe may become stuck.
Differential sticking usually occurs when the pipe has been motionless for a period of time, e.g., when making a connection or during surveying. Differential sticking can be a particular problem when drilling depleted reservoirs because of the associated high overbalance.
The force required to pull differentially stuck pipe free depends, inter alia, upon the following factors:
(a) The difference in pressure between the borehole and the formation. Any overbalance adds to side forces which may exist due to the deviation of the hole.
(b) The surface area of pipe embedded in the wall cake. The thicker the cake or the larger the pipe diameter, the greater this area is likely to be.
(c) The bond developed between the pipe and the wall cake is a very significant factor, being directly proportional to the sticking force. This can include frictional, cohesive and adhesive forces. It tends to increase with time, making it harder to pull the pipe free.
Differential sticking may be distinguished from other forms of sticking, such as mechanical sticking. Mud circulation is not interrupted as there is no obstruction in the hole to stop the flow, as would be the case for pipe stuck due to hole bridging or caving. It is not possible to move or rotate the pipe in any direction.
When a pipe sticks the driller usually tries to free it by mechanical movement, e.g., by pulling, jarring or, if the pipe was moving immediately prior to sticking, trying to move it in the opposite direction. Frequently this fails to release the pipe and there is, of course, a limit to the force which can be applied, since too much force could fracture the pipe and make the situation worse.
If the pipe remains stuck it is then sometimes the practice to apply a pipe release agent.
Pipe release agents are chemically active mixtures, which may be oil or water based, which are placed over the stuck region in an attempt to free the pipe if working the pipe has failed to release it. These are believed to act by attacking the mud filter cake. They are positioned by pumping them down the hole to the stuck region in the form of a slug, known as a pill. The pill generally contains sufficient material to cover the stuck zone and extend slightly beyond it over a total area of 1.52 times the area of the stuck zone. The pill volume necessary to achieve this is usually about 100 bbl. Pills frequently are left to soak until the pipe is free or attempts to free the pipe are abandoned.
Pipe release agents are generally sold as proprietary blends by companies trading in this area, frequently without divulging their chemical components. However, some pipe release agents are based on asphaltenic compounds and some on glycols, glycerols, alcohols and fatty acids and derivatives of such compounds. Traditionally, pill density has been increased in a similar manner to muds, using solid particulate weighting agents such as barite, with viscosifiers to prevent settling, to preferably match the density of the spotting fluid to the density of the drilling fluid which had been used up to the time of the pipe being stuck. For example, if the drilling fluid had a density of 14 pounds/gal (ppg), then the spotting fluid would be weighted up, for example, with barite, to result in a spotting fluid density of 14 ppg.
There are many such pipe release agents known in the prior art. In those cases where environmental concerns do not prohibit or discourage their use, diesel oil has been used with a fair amount of success.
For many years, asphalt-based spotting fluids such as the product BLACK MAGIC SFT marketed by Baker-Hughes was also used with a fair amount of success. In those cases where asphalt, diesel oil, mineral oil or other oil-based spotting fluids cannot or should not be used, many other commercially available, environmentally-friendly spotting fluids have been used in an attempt to free stuck pipe, oftentimes, only with limited success.
For example, the use of polyalphaolefin as a lubricant/spotting fluid is disclosed in David O. Trahan, et al. U.S. Pat. No. 4,876,017, such material being marketed by The Coastal Mud Company under the trade name COASTALUBE.
James R. Hayes, et al. U.S. Pat. No. 5,002,672 is another example of a more environmentally-friendly spotting fluid (marketed by Turbo-Chem International, Inc. of Lafayette, La.) comprising the use of a glycerophosphoric acid ester and a polyacyloxy polycarboxylic acid ester of mono and/or diglycerides, in combination with other viscosifiers and sealing agents.
Heinz Mueller, et al. U.S. Pat. No. 4,964,615 discloses yet another spotting fluid using fatty acid alkyl esters or mixtures of esters.
Many other commercially-available spotting fluids are known in the art such as, for example, the COASTAL SPOT spotting fluid (a polyol ester) marketed by the Coastal Mud Company, Abbeville, La., the ENVIRO-SPOT, the PETROFREE Ester and the BARO-SPOT spotting fluids marketing by Baroid Drilling Fluids, Inc. of Houston, Tex., the CESCO A-25 SPOT product marketed by Cesco Chemicals, Inc. of Lafayette, La., the PIPE-LAX product (a glyceride) marketed by M-I Drilling Fluids, Inc. and the VL-250 product (a blend of terpenes-cylic C.sub.10) marketed by Integrity Industries. The ENVIRO-SPOT product is a blend of emulsifiers, lubricants, gellants and other materials. The BARO-SPOT product is a blend of surfactants, lubricants and viscosifiers. GILSONITE is marketed by Cesco Chemicals, Inc. at Lafayette, La. SURFYNOL 440, a C.sub.14 Accylene Polyol, is marketed by Air Products, Inc. BXR-2000 is a polyglycol marketed by Baroid Drilling Fluids, Inc. MONA 939 is a phosphate ester marketed by Mona Industries, Inc.
In SPE Paper No. 22550, presented in Dallas, Tex. on Oct. 6-9, 1991, there is the suggestion on its pages 159-160, that a single spot mixture of 40% sodium chloride brine (20 wt % salt) with glycerol (60 vol %) results in rapid pipe release and a cracked filter cake, and concluded that "These results demonstrate that differentially stuck drill pipe can be released with oil-free fluids, providing encouragement that an effective non-oil spotting fluid can be formulated." Also on page 159 of SPE Paper No. 22550, there is a discussion of using a single spot calcium chloride/calcium bromide as a pipe release agent, all as shown further in Table 7 of the paper.
It is also known (the KOPLUS LL material marketed by Gait International of Dublin, Ireland) to use citric acid in combination with organic or inorganic salts (including potassium chloride) to free stuck pipe.
However, each of the available more or less environmentally-safe spotting fluids has had only limited success in freeing stuck pipe.
It is therefore the primary object of the present invention to provide a more reliable method of freeing stuck pipe.
It is another object of the present invention to provide a new and improved combination of materials which can be used to free stuck pipe.